1. Field of the Invention
The present invention relates to a method and apparatus for operating an underwater power plant, especially a power plant standing freely in an ocean current, dealing especially with a method and apparatus for adjustment to the change in the inflow direction caused by the tide and for moving towards a maintenance position.
2. Description of the Related Art
Free-standing, immersed power generation plants can be used economically from flow velocities of 2 to 2.5 meters per second. Water turbines with a propeller-like structure, for example, are fastened for this purpose to support structures for taking up kinetic energy from the ambient current. Depending on the water depth and the nature of the ground of the ocean, the support structures can be arranged, for example, as fixed pillars which are placed on a foundation in the ground. As an alternative, the support structures are merely anchored to the ground of the ocean by means of cable or chain systems and form floatable units floating at a certain depth in the water.
The relevant state of the art is known from DE 80 91 79 C, GB 23 11 566 A, U.S. 2006/0153672 A1 and U.S. Pat. No. 7,105,942 B2.
Characteristic for tidal currents is the regular change of direction of the current with ebb tide and flood tide. It is, therefore, necessary for efficient utilization of energy to arrange a respective free-standing underwater power plant driven by tidal currents in such a way that the up-take of energy by the water turbine from the current is possible with different inflow directions. For this purpose, two directions of current which face opposite of each other are principally relevant in ebb tide and flood tide.
In the simplest of cases, an anchored system which rotates freely about an anchoring point can be used for adjustment to the different directions of current. The disadvantageous aspect is, however, that a large circle of motion cannot be avoided in most cases and, therefore, such systems cannot be combined efficiently into an energy park with several underwater power stations. If stationary plants are used instead, and especially stations standing on a foundation, an adjustment to changeable directions of current will then be simple when the turbine is arranged as lee-side runners. In this case, a link joint is used for linking a spacer element to the support structure, with the turbine being fastened to the end on the off-stream side of the spacer element. This is typically a propeller-shaped turbine with two or more turbine blades which are arranged in the manner of rotor blades.
The disadvantageous aspect in a lee-side runner is that a certain shading effect by the support structure (tower shadow) cannot be avoided. Advantageous for reasons of efficiency is the use of a current-side runner, i.e. an arrangement of the turbine upstream and with a certain distance to the support structure, to which the same is fastened at least indirectly. However, there is no possibility for a current-side runner to make any passive readjustment with a changeable direction of inflow, so that the position of the turbine to the direction of the current needs to be guided actively. The drives known for this purpose usually comprise complex rotational concepts in the area of the tower. These are disadvantageous in view of the desire to produce underwater power stations requiring as little maintenance as possible because the additionally used gear and motor components lead to an increased probability for failure. Accordingly, there is higher expenditure for regular inspections, which themselves pose difficulties in view of the limited accessibility of underwater power stations.
Solutions have been sought to make the simplest possible adjustment of a free-standing tidal power station to an inflow from different directions. One proposal was to use a rotor-like turbine and to enable inflow from opposite directions via blade angle adjustment. Although this measure allows omitting the rotation of the entire turbine into the current, the problem of increased need for maintenance will be shifted due to additional moved components and the actuators associated with the same to a turbine-side apparatus, which is also susceptible to errors.
It is further desirable, in addition to adjustment to the direction of the current, to enable moving the turbine, and preferably a generator unit connected with the same, to a maintenance and mounting position. This requires, on the one hand, twisting out of the current and, on the other hand, in most cases a lifting of the turbine-generator unit to the water surface. Accordingly, the turbine will assume a certain relative position to the ambient current for initiating an inspection, so that even in this case the problems as explained above will occur.
What is needed in the art is a method and an apparatus which enable the turbine of a free-standing underwater power station to assume a relative position to the ambient current. Power up-take from the ambient current should be especially possible in the case of opposing main directions of current. Moreover, it is desirable to set an additional turbine position with reduced inflow, e.g. for initiating service measures. Further needed is a method which is carried out by means of a sturdy apparatus adjusted to long service intervals. Especially preferable is a substantially maintenance-free apparatus.